A carbon nanotube is a strong candidate for a wiring material to be used in a next-generation semiconductor device which can replace a currently employed Cu wiring due to its good electric conductivity (low electric resistance), good thermal conductivity (high heat dissipation) and high current density resistance (high electromigration resistance). To that end, the carbon nanotube is required to have a specific long length and specific orientation. For example, when a carbon nanotube is employed as a via wiring, the carbon nanotube has to be arranged perpendicularly to a substrate surface in high density.
As a carbon nanotube growing method, a plasma CVD may be employed, wherein hydrocarbon molecules and the like in a source material are excited/decomposed by using a high-energy source such as plasma, and active species thereof and catalytic metal react with each other to thereby grow carbon nanotubes. For example, Japanese Patent Application Publication No. 2007-252970 (JP2007-252970A) discloses a plasma CVD as the carbon nanotube growing method, wherein a catalyst layer made of a transition metal such as Ni, Fe or Co is formed on a substrate, and a carbon nanotube film is formed on the catalyst layer at a processing temperature of 600° C. by employing the plasma CVD using a carbon containing gas and a hydrogen gas. JP2007-252970A describes that, when a catalytic metal is provided in the form of fine particles, radicals in a plasma generated by using the carbon containing gas and the hydrogen gas are applied to the surfaces of the fine particles of the catalytic metal to thereby activate the surfaces of the fine particles in order to prevent deterioration of the catalyst activity due to oxidation of the surfaces of the fine particles.
In the meantime, a thermal CVD is also well known as one of the carbon nanotube growing methods, wherein hydrocarbon molecules of a source material are thermally decomposed on the surface of a catalytic metal to grow the carbon nanotube. For example, Japanese Patent Application Publication No. 2007-261867 (JP2007-261867A) discloses a thermal CVD as the carbon nanotube growing method, wherein a carbon nanotube film is formed on a substrate on which fine particles serving as a catalyst are formed at a temperature in a range between 800 and 1000° C. by the thermal CVD by using a hydrocarbon-based gas as a source material. JP2007-261867A describes that a segmentation process wherein a catalytic metal (e.g., Fe) thin film formed on a substrate is converted into catalytic metal particles by being heated to a temperature in the range between 800 and 1000° C. under an oxygen atmosphere (e.g., atmospheric state) to be dissolved and then cooled; and a re-segmentation process wherein the catalytic metal particles are converted into catalytic fine metal particles by being heated at a temperature in the range between 800 and 1000° C. under an oxygen atmosphere (e.g., atmospheric state).
In the thermal CVD disclosed in JP2007-261867A, it is required to heat the substrate on which a catalytic metal is formed to a high temperature that is equal to or higher than 800° C. This may cause diffusion of impurities and/or increase in thermal budget, which makes it difficult to apply the thermal CVD to the process of manufacturing semiconductor devices.
In the plasma CVD disclosed in JP2007-252970A, the carbon nanotube can be formed at a heating temperature that is lower than that required for the thermal CVD. Since, however, it is necessary to heat the substrate to a temperature of about 600° C. in the plasma CVD, an adequate heat resistance is required for the substrate and/or a material film formed on the substrate. This makes it difficult to apply the plasma CVD to a plastic substrate, for example.